A porous member with penetrating channels for fluid flow therethrough and a method for producing the member The invention refers to a porous member of temperature-resistant material having channels extending therethrough through which a fluid (gas and/or liquid) may pass, and to a method for producing this member.
Such members are used, for example, as chemical mixers. The conventional chemical mixers consist of corrugated sheets of steel or ceramic fabrics. These abutting steel sheets or ceramic fabrics are comprised into units with crossing channels forming between the steel sheets or the fabrics. Sometimes the steel sheets are perforated so that a fluid flow passing through this chemical mixer member is homogenized, thereby mixing the fluid. For reasons of production, the individual mixer members cannot be produced with more than a certain maximum length. Thus, for producing longer mixers, a plurality of such mixer members are arranged in line. This increases the cost of the production process.
It is the object of the invention to provide a porous member with penetrating channels through which fluid may flow, wherein the homogenization and mixing of the fluid is improved, and to provide simplified methods of producing such a member, substantially independent of the length of the porous member.
The present porous member of temperature-resistant material has an inlet surface and an outlet surface The fluid flowing through the porous member enters the porous member at the inlet surface and leaves the porous member at the outlet surface so that flow direction runs from the inlet surface towards the outlet surface. According to the invention, the porous member is provided with first channels extending under an acute angle to the flow direction. In the following, an acute angle is an angle other than 0xc2x0. Due to the porosity of the member, the channels penetrating the member are in communication so that a mixing of the fluid flowing through the porous member occurs. Since the channels form an acute angle with the flow direction, a part of the fluid flows through the channels and a part of the fluid flows in the flow direction through the porous portions, i.e., the porous walls between the channels immediately in the flow direction. Thus, the fluid (or several fluids) is mixed and homogenized so that a homogenized fluid exits from the outlet surface of the porous member.
Since the porous member is temperature-resistant, it is suited for mixing hot fluids. In doing so, due to the present structure of the member, no local overheating occurs that could damage or destroy the porous member.
Moreover, the present porous member is suited for filtering fluids.
Here, particles are deposited on the walls of the pores. Due to the temperature resistance of the member, which is made, for example, from high temperature-resistant ceramics such as zirconia oxide or silicon carbide, the member used for filtering may be cleaned by burning, whereby the residual filtered matter is combusted. This does not damage the filter. In particular, the present porous member is simultaneously suited for mixing and filtering fluids.
Depending on the shape of the porous member and the position of the inlet and outlet surfaces, the flow direction varies. For example, in a cylindrical member whose end faces are the inlet and outlet surfaces, respectively, the flow direction is straight. In a bent or otherwise shaped porous member, the center line of the member corresponds to the flow direction so that the flow direction varies in the longitudinal direction of the member.
Preferably, besides the first channels, the porous member has second channels that are also arranged under an acute angle to the flow direction and, in addition, are arranged under an angle to the first channels, i.e. an angle other than 0xc2x0. Such crossing channels that may penetrate each other at least partly, improve the homogenization and mixing of the Fluids. A further improvement may be achieved by varying the angle of the channels to the flow direction along the extension of the channels. In a cylindrical porous member with a continuous flow direction over the length of the member from the inlet to the outlet surface, the channels are wavy or zigzag-shaped, for example. Thus, a portion of the fluid flowing through the member that is sufficient to homogenize the fluid, flows through the porous portions.
The channels of the porous member are preferably arranged in rows, at least one row of first channels and a row of second channels being provided, and the channel rows being arranged alternating side-by-side and in parallel.
In a preferred embodiment, the porous member is cylindrical and the first and/or second channels extend from the inlet surface to the outlet surface of the member. Here, cylindrical means a longitudinal member with parallel end faces having the same, but an optional contour. Depending on the conditions of the device in which the porous member is used, the contour may be a curve or a polygon, for example.
Besides the usefulness of the porous member as a chemical mixer for mixing fluids and as a filter for filtering particles from fluids, the present porous member can also be used, in particular, in the combustion chamber of a pore burner.
Pore burners have a housing with an inlet and an outlet, a mixture of gas and air flowing into the pore burner through the inlet and the flue gases being exhausted from the pore burner through the outlet. Prior to ignition, the gas-air mixture flows in the flow direction of the pore burner and through a backfire means. As the backfire means, a conventional flame retention baffle or a plate with holes may be provided, for example. The backfire means prevents the gas-air mixture burning behind the backfire means, seen in the flow direction, from backfiring towards the inlet opening. The backfire means is followed by the combustion chamber in which the gas-air mixture is ignited by an ignition means and burned therein. Pore burners are characterized in that the combustion chamber accommodates a heat-resistant porous material in which the gas-air mixture is combusted. Thus, a more uniform combustion of the gas-air mixture is obtained so that within a large effective range of the pore burner, only small amounts of pollutants such as NOx or CO are produced during combustion. Using the present porous member in the combustion chamber of the pore burner, the emission of pollutants is reduced further. Thus, even at one thirtieth of the nominal power of the pore burner, a clean combustion is obtained. The pore burner is particularly suited for use in heating installations.
A further use of the porous member, according to the invention, is its implementation as a heat accumulator. Due to the porous structure of the member and the channels extending therethrough, the fluid flowing through the porous member is distributed homogeneously over the cross section of the member so that, when hot fluid flows through the member, the heat is received homogeneously by the porous member. When cooler fluid passes through subsequently, the heat uniformly transferred from the porous member to the fluid so that a uniform heating of the fluid occurs. Therefore, the present porous member is very well suited for use as a short- and medium-term heat accumulator.
In particular, the porous member is suited for use as a heat accumulator in regenerative radiant burners. Regenerative radiant burners serve to heat material, for example, steel ingots, by thermal radiation. Here, two radiant burners operating at intervals are employed. Each burner is provided with a porous member as the heat accumulator. Further, a blower or suction device is provided which, at intervals, either supplies the burners with a mixture of fuel and air or fresh air or exhausts the flue gases. During the first cycle, the blower directs fresh air or a mixture of gas and air through the heat exchanger of the first burner to the combustion head, where the fresh air is either mixed with fuel and ignited or the mixture of fuel and air is ignited directly. In the first cycle, the burner flame of the first burner heats the material located above the burner by radiant heat. In the meantime, the flue gases are exhausted through the heat exchanger of the second burner, heating the same up. To this end, the same blower or a separate suction device can be used. In the second cycle, the function of the two burners is interchanged so that fresh air or a gas-air mixture is passed through the heat exchanger of the second burner and heated up in the process. The heated gas-air mixture is ignited in the combustion head of the second burner. In the second cycle, the material is heated by the radiant heat of the flame of the second burner and the flue gases are exhausted through the heat exchanger of the first burner. By providing the heat accumulators in each of the two burners, the gas-air mixture or the fresh air is pre-heated so that the efficiency of the radiant burners is increased significantly. The use of the porous members of temperature-resistant material, preferably ceramics, as provided by the invention, prevents local damages to the heat exchanger even at very high temperatures. Using the porous member as a heat accumulator, it is possible to pre-heat the fresh air to about 1000xc2x0 C. Moreover, the porous members serve as exhaust gas filters.
For manufacturing the porous member of temperature-resistant material, in particular ceramics, with channels extending through the member, the invention proposes a method, wherein, according to a first variant
at least one insert member is placed in a mold, the insert member defining the course and the shape of at least one channel,
a foamed plastic material is introduced into the mold so that the at least one insert member is embedded in the foamed plastic material member which is flexible after curing,
the foamed plastic material member is removed from the mold and the at least one insert member is removed from the foamed plastic material member,
the surface of the foamed plastic material member is wetted with a curable temperature-resistant wetting material, and
the foamed plastic material of the foamed plastic material member wetted with wetting material is removed by heating the same so that a porous member made from the wetting material and adapted to be flown through by a fluid is obtained.
It is the principle idea of the present method to first provide a flexible foamed material structure with channels running therethrough. According to the invention, this is done by providing foamed plastic material around an insert member representing the course of the later channel and by embedding the same therein. Alternatively, a plurality of such insert members may be used. The insert members may have a structure changing in all three dimensions so that eventually not only straight, but also curved, wavy or zigzag-shaped channels may be produced. The foamed plastic material is flexible after curing so that the insert members can be pulled from the foamed plastic material. Thereafter, a foamed plastic material member is left through which extend one or a plurality of straight or curved channels, the channels being in communication due to the porous structure of the foamed plastic material. The foamed plastic material suitably is an open-cell or a closed-cell material. In particular, polyurethane is used as the foamed plastic material.
After the insert member(s) has (have) been removed from the foamed plastic material member, the foamed plastic material member is wetted with a wetting material. This wetting can be imagined as a wetting of the entire surface of the structure of each foamed plastic material member with the wetting material. Preferably, the wetting material is slip, in particular ceramic slip. It is of general importance that the wetting material is cured so that after curing a self-supporting wetted foamed plastic material member is obtained, the dimensional stability and the self-supporting ability thereof being provided by the wetting material. Thus, the porous member thus formed includes, on the one hand, the channels with their inner walls wetted with the wetting material and, on the other hand, the connections between adjacent channels also wetted on their inner walls. Subsequently, this foamed material member wetted with hardened wetting material is heated to a degree that the foamed plastic material is removed by burning. An alternative method of removing the foamed plastic material is the evaporation resulting, for example, from a chemical reaction with a corresponding process gas.
In the manner described above, porous members with channels and with optional length may be made, even when the channels have undercuts or similar three-dimensionally varying paths. Such a member may be employed as a catalyst, if, for example, a catalytically active layer is applied. With or without this additional layer, it may anyway be used as a chemical mixer homogenizing a fluid mixture flow passing through, thereby mixing the fluids. It is a further advantage of such a mixer (a porous member with channels running therethrough) that it has only a low flow resistance. Of course, one would obtain a fluid flow rate with a porous member, causing, however, a much greater flow resistance than with a porous member produced according to the above method. Likewise, the present porous member may be used as a filter. Since the porous member is made of temperature-resistant material, the filter may be cleaned by burning.
According to a second variant of the invention, the porous member of temperature-resistant material, in particular ceramics, penetrated by channels and adapted to be flown through by fluid may be produced by
producing a wavy flexible mat of foamed plastic material,
winding the mat into a foamed plastic material member,
wetting the surface of the foamed plastic material member with a curable temperature-resistant wetting material, and
removing the foamed plastic material of the foamed plastic material member wetted with wetting material by heating the same, so that a porous member made from the temperature-resistant wetting material and adapted to be flown through by a fluid is obtained.
With this production variant, a wavy flexible mat of open-cell or closed cell foamed plastic material is produced which is then rolled into a (wound) member. The foamed plastic material member thus formed is penetrated by channels that are formed between the valleys and the peaks of adjacent windings of the wavy flexible mat. For example, the wound member retains its wound structure by using an adhesive.
The insert members may be rigid or, preferably, flexible and/or inflatable. After the foamed plastic material member has been produced, the insert members inflated until then may be deflated or relaxed so as to be removed from the foamed plastic material member. Should the insert members not be inflatable, but flexible, they may readily be pulled from the foamed material due to their flexibility.
After the forming of the wound member, another wetting with temperature-resistant wetting material is performed, the material being allowed to cure. Thereafter, the member thus made is heated to remove the foamed plastic material. The advantages to be obtained with the porous member of this variant are identical to those described for the first variant of the invention. In addition, the manufacturing process of the second manufacturing variant is simpler since no insert members are embedded in the foamed plastic material member that have to be withdrawn from the cured, but still flexible foamed plastic material.
It is true for both variants of the invention that the wetting material preferably is slip, in particular ceramic slip. The wetting step is performed by drenching the foamed plastic material member with the wetting material. These methods are known per se from the production of ceramic foams. The ceramic slip is cured by burning. At the same time, the foamed plastic material is removed by evaporation.
Further, it is true for both above described variants of the invention that the foamed plastic material member wetted with the wetting material is cured. Since the foamed plastic material member is still flexible while the wetting material is not yet cured, it may be placed in bent or otherwise shaped molds (molds for connector pipes, for example, or the like) to then cure in these molds. Thus, the finished product has a shape that allows a compact mounting in a mixer or filter assembly or a pipeline of a mixer or filter assembly. Finally, it is suitable to make the insert members and the mold from material inert to the foamed material or to provide it with a coating of inert material.
For producing a porous member of temperature-resistant material, in particular ceramics, adapted to be passed through by fluid and being provided with channels extending therethrough, the invention proposes the following method as a third variant, wherein
at least one foamed plastic material member with a top and a bottom surface is formed from a flexible foamed plastic material,
the foamed material member being sheared in a first direction about a shear angle by being subjected to a first shearing force,
from the top and/or the bottom surface, first channels are formed in the foamed plastic material member thus sheared, the channels being formed under an angle to the normal of the top and/or bottom surface, this angle being different from the first shear angle,
the first shearing force is relaxed and the foamed plastic material member restores itself,
the surface of the foamed plastic material member is wetted with a curable temperature-resistant wetting material, and
the plastic material of the foamed plastic material member wetted with the wetting material is removed.
The essential idea this method is based on is to first provide the flexible foamed plastic material structure with channels extending therethrough. For an improved mixing of the fluid passing through the member, these channels extend obliquely to the axial extension of the member. One could form these oblique channels under an angle other than 90xc2x0 to the top or bottom surface of the foamed material structure or the foamed material member. This, however, makes the production process more difficult, which is due in particular to the flexible structure of the foamed material member. Therefore, in this third variant of making the present porous member, it is proposed to shear the foamed material member, i.e., to subject the foamed material member to shearing forces. Now, the channels may be formed under an angle of 90xc2x0, in particular, to the top or bottom surface of the sheared foamed material member. When the shearing force acting on the foamed material member is subsequently relaxed so that the foamed material member is in its relaxed state, the channels extending through the foamed material member are oblique.
In the manner described above, first channels extending in a first direction may be formed in a foamed plastic material member. Subsequently, the foamed material member, according to the first and second variant, is wetted with temperature-resistant wetting material and the foamed plastic material member is removed.
When the foamed plastic material member is sheared in another direction after the forming of the first channels, which direction is preferably opposite to the active direction of the previously applied shearing force, second channels may be formed in the foamed material member extending through the foamed material member in a direction different from that of the first channels. Thus, two groups of channels run through the foamed plastic material member, having different orientations.
Depending on the magnitude of the shearing forces and their effective directions, channels with different degrees of inclination may be formed in a formed plastic material member. The orientation of the channels also depends on the angle under which they are formed in the sheared foamed member.
The process steps of shearing the foamed plastic material member and of forming channels can also be performed simultaneously. To this end, for example, a punching tool may be set on the top or the bottom surface of the undeformed foamed member which is not yet subjected to shearing forces. As soon as the punching tool contacts one side of the foamed member, it is displaced relative to the opposite side of the foamed plastic material member so that shearing forces act on the foamed member. The punching is performed either upon reaching the desired shear angle or already during shearing. Thus, it is sufficient to displace the punching tool during punching, relative to the opposite side of the foamed member so that shearing forces act on the foamed member.
For the channels to be punched in the foamed member to have a possibly circular section, the foamed member may be pressed prior to or after the punching, whereby it is compressed.
In a fourth variant of the process, the channels are formed in a foamed member without shearing the same. In this variant, first channels are formed in an outer surface of a foamed member. Subsequently, the foamed member is cut, the cutting surface extending under an acute angle to the longitudinal extension of the first channels. A foamed member thus cut can be processed further, until a plate material is obtained having parallel top and bottom surfaces, one surface being defined by the cutting surface.
In the variant described above, second channels may be formed in the cutting surface after the cutting of the foamed plastic material member, whereupon the foamed plastic material member is cut again such that this cutting surface extends under an acute angle to all of the channels. When the foamed plastic material member thus cut is processed further so that a plate material or foamed material blocks are obtained, a foamed material structure is produced that is run through by channels extending oblique to each other and that has parallel top and bottom surfaces, at least one of which is defined by the (second) cutting surface. Thereafter, the foamed plastic material member is again wetted with wetting material, as described above, and the foamed plastic material member is removed.
In the most general form, this advantageous production method according to the third and fourth variants provides a foamed material structure through which oblique channels extend, wherein groups of these channels extend in parallel and the channels may be subdivided into a plurality of groups of channels with different relative orientations.
Suitably, a plurality of thus produced foamed material members are superimposed, wetted and cured, the foamed plastic material member being removed during curing, so that a member can be produced through which fluid may flow and which has properties of a mixer, all this without restrictions in length incurred by the manufacturing process. The individual foamed material members are preferably made as a plate material of random geometric shape with parallel top and bottom surfaces.
The channels are preferably formed by punching the foamed material member. Such a punching tool comprises two pressing members frictionally contacting the top and bottom surfaces of the foamed material member. When these two pressing members have been brought into frictional engagement with the foamed plastic material member, at least one pressing member is moved relative to the other so that the foamed plastic material member arranged therebetween is sheared. Now, the punching tool can be moved into the foamed plastic material member. Here, it is feasible to compress the sheared foamed plastic material member by moving the pressing members toward each other, such that the holes may be formed by means of punching tools.
It is further suitable to have perforated pressing members so that the punching tools may be advanced through the holes into the foamed plastic material member. In the state in which the foamed plastic material member is punched, the holes of the two pressing means should be flush.
Thus, the third and fourth variants of the present method provides for the production of a porous member through which a fluid can flow, the member comprising either a single foamed plastic material member or several superposed foamed plastic material members which or each of which has one channel or a plurality of channels extending under a common oblique angle or under different oblique angles. Due to the porous structure of the foamed plastic material member, these channels are interconnected. When using a plurality of foamed plastic material members, it is suitable to have different, in particular oppositely directed, orientations of the adjoining channels of adjacent foamed material members.
Suitably, the foamed plastic material is an open-cell or a closed-cell material. In particular, polyurethane is used as the foamed plastic material.
When a plurality of adjoining foamed plastic material members are provided with wetting material, it is suitable to interconnect the foamed plastic material members before wetting. Here, it is feasible to weld the foamed plastic material members together by heating their contact surfaces. An alternative to this connection is to couple adjacent foamed plastic material members by means of the cured wetting material.
Since the foamed plastic material member or the group of successive foamed plastic material members is still flexible with the wetting material not yet cured, it can be placed in curved or otherwise shaped molds (such as manifold molds or the like) to be cured in these molds by burning. Thus, the finished product is given a shape that allows for a compact installation in a mixer device or a tubing of such a mixer device.
The present invention allows to produce ceramic mixers that are characterized by an excellent mixing of the fluid flowing therethrough. When a porous member produced according to the invention is used, for example, to mix the combustion gases of an internal combustion engine, as, for example in a motor vehicle, a catalyst also used therein may be arranged closer to the outlet valves of the internal combustion engine than was possible up to now. The catalysts presently used, in particular, in vehicle production, are supposed to have a minimum distance from the outlet valves of the internal combustion engine, because a sufficiently well mixture of the combustion gases occurs only at a certain distance after the outlet valves so that their cleaning by the catalyst is satisfying. A mixer produced according to the present invention, however, makes it possible to reduce the distance between the catalyst and the outlet valves of the internal combustion engine. This is advantageous with a view to the cleaning of the combustion gases, since the combustion gases now flow into the catalyst at a higher temperature.
In the present method, it should be underlined as being particularly advantageous that for producing the foamed plastic material members, the conventional production methods for foams may be used. The foams, mostly coming as blocks, merely have to be processed to plate material which will then be sheared in accordance with the present invention so as to form the channels. The production of block foam is rather economic so that, after all, also the rigid members to be flown through by a fluid can be made at relatively low cost, making use, in particular, of the well-known technology of foam production. In particular, no special molds are required for the foam production. Thus, the production process of the present invention uses a semi-finished material, i.e. foamed plastic plate material which is available at extremely low cost. Also the further method steps, especially the forming of the oblique channels is done, according to the present invention, in a simple and, in view of production technology, economic manner.
The following is a detailed description of the preferred embodiments of the present porous member and the methods of its production, made with reference to the accompanying drawings. In the Figures: